[Bug libstdc++/59872] [4.9 Regression] Cannot move std::map with move-only mapped_type

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59872

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I don't want to use SFINAE here, I'll fix it the same way it's done in the
other containers, tag dispatching using
  std::integral_constant<bool, _Alloc_Traits::_S_always_Equal()>